This invention relates to carbon fibers having a novel fiber structure, which can provide a composite material having an ultrahigh strength in comparison with conventional carbon-fiber reinforced composite materials, and more particularly, carbon fibers which exhibit an ultrahigh strength of at least 500 kg/mm.sup.2, more preferably about 600 kg/mm.sup.2 or greater in terms of the resin-impregnated strand strength.
Carbon fibers have already been industrially produced and widely employed for use as reinforcing fibers for composite materials utilizing remarkable mechanical properties, particularly the specific strength and the specific modulus of carbon fibers, but in connection with such known composite materials, particularly those for use in the field of aeronautical and/or aerospace industries, it has been increasingly strongly demanded that an enhancement is met of the strength of carbon fibers.
To cope with such demand, there have already been some propositions made, but the carbon fibers pertaining to those propositions do not necessarily exhibit such a mechanical strength which can fully satisfy the demand. Particularly, there still lies a problem such that even if the mechanical strength of the carbon fiber itself can be improved, the improved mechanical strength of the carbon fiber is not made contributive to an enhancement of the mechanical strength of a composite material, that is to say, generally low are degrees of utilization made of the strength of carbon fibers. Besides, the known or proposed processes for the production of carbon fibers involve complex steps and/or difficulties in controlling production conditions, and they are problematic as an industrial production method.
Also, in the production of composite materials comprising carbon fibers as their reinforcing fibers, whereas it is sought for to obtain an improvement in or relating to the ease of handling of carbon fiber bundles (multifilaments) in the process of forming the composite material and also in the resin-impregnated strand strength of carbon fibers, the known carbon fiber bundles do not afford a desirable ease of handling; for example, they easily tend to undergo breakage, fluffing and so forth in the processes of winding-up thereof and/or preparation of prepregs, and their mechanical strength is at highest about 570 kg/mm.sup.2 in terms of the resin-impregnated strand strength and at highest about 520 kg/mm.sup.2 in terms of the average filament strength.
Furthermore, with the known carbon fibers of which the mechanical strength is improved as above, they generally have a high resin dependency, that is to say, their resin-impregnated strand strength varies depending upon the kind of the matrix resin used in the composite. Thus, they have a defect that the mechanical strengths thereof, even though improved, are not sufficiently translated in composite materials due to their high dependency on a matrix resin.
Carbon fibers are usually subjected to an electrolysis treatment to generate functional groups on the surface thereof and improve the adhesion of the fiber to the matrix resin and the interlayer shear strength (ILSS) of a composite material prepared from the fiber (see, for example, Japanese Patent Publication No. 20033/1980). However, this treatment is only to improve the adhesion of the carbon fiber to the matrix resin, and cannot be expected to improve the tensile strength of the fiber itself or a composite material prepared therefrom.
On the other hand, in order to improve the strength of carbon fibers itself, there have been proposed processes comprising immersing carbon fibers in an inorganic acid such as concentrated sulfuric acid, nitric acid or phosphoric acid for a long time to etch the surface of the fiber, and subsequently subjecting the fiber thus etched to a heating treatment in a high-temperature inert atmosphere to remove the functional groups formed on the surface of the fiber by the above-mentioned inorganic acid immersion treatment (see, for example, Japanese Patent Application Laying-open Publication No. 59497/1979, and Japanese Patent Publication No. 35796/1977). According to the disclosure the Japanese patent application Laying-open Publication No. 59497/1979, such an etching treatment serves to remove the surface layer of the fiber together with flaws formed on the surface of the fiber in the course of the process of production of the carbon fiber to improve the mechanical strength of the carbon fiber.
One of the inventors of the present invention found that, in the process comprising subjecting carbon fibers to a chemical oxidation treatment and heating the oxidized carbon fibers in an inert atmosphere to remove the functional groups on the fiber surface, the average filament strength of the carbon fibers obtained is largely improved, by suitably selecting the treatment conditions under which the surface layer region of carbon fibers can be selectively rendered amorphous, and proposed a process involving such treatment conditions (Japanese patent application Laying-open Publication No. 214527/1983). However, despite the treatment conditions so specified, it turned out that the resin-impregnated strand strength of the treated carbon fibers has a high resin dependency, thus presenting a problem in practicability.
More specifically, it was found that, when carbon fibers are subjected to a severe treatment by which the fiber surface becomes etched in spite of an extremely high chemical resistance thereof, the obtained carbon fibers are damaged not only in the surface layer region but also, in some cases, in an inner region, thus not always resulting in an improvement in the mechanical strength, and that, despite any improvement in the mechanical strength of the carbon fiber, the resin-impregnated strand strength thereof is not improved, thus failing to contribute to any improvement in the tensile strength of a composite material prepared therefrom. Particularly, it was turned out that, as the mechanical strength of carbon fibers to be subjected to the etching treatment with an inorganic acid is increased, the improvement in the strength of carbon fibers attained by this treatment is reduced to thereby provide little expectation of any marked improvement in the resin-impregnated strand strength, and that, in addition, the mechanical strength of a composite material prepared from carbon fibers subjected to the above-mentioned treatment has a higher resin dependency.